This project involves the development of a system to measure blood PO2 and PCO2 in extracorporeal circuits for extended periods without blood loss. In addition, blood oxygen saturation measurements can be made simultaneously. Nanomole amounts of carbon dioxide and oxygen are extracted from the circulation with a special flow through probe. Oxygen is analyzed electrometrically and carbon dioxide is measured by calorimetery of the reaction with carbon dioxide and lithium hydroxide. In vitro tests have shown the system to be stable for at least one week of continuous measurement and ex vivo tests using a sheep as an experimental animal have demonstrated acceptable performance for more than 72 hours.